1. Field of the Invention
This invention relates to an electrophotographic method and apparatus, and more particularly to an electrophotographic method and apparatus which enables formation of electrostatic images well representing the tone of original image to be effected on a photosensitive medium having a photoconductive layer.
2. Description of the Prior Art
As the basic electrophotography, there is known the Carlson method disclosed by C. F. Carlson in U.S. Pat. No. 2,297,691. In this method, use is made of an electrophotographic sensitive medium having a photoconductive layer such as amorphous selenium or the like on the surface of a conductive substrate, and the surface thereof is uniformly charged in the dark and then exposed to image original, whereby the surface charge of the photosensitive medium illuminated by light is discharged to thereby form an electrostatic latent image. Such electrostatic latent image is developed by toner particles and then transferred to transfer medium or the like for utilization, as is well-known.
Generally, the range of the brightness reproduction for an original by the above-described Carlson method is of the order of 0.6 to 0.8 and for example, where amorphous selenium is employed, the relation of the latent image potential to the density of the original image is such as shown by full line in FIG. 1 of the accompanying drawings. In fact, the copy density with respect to the original density cannot achieve complete reproduction of density as is shown in FIG. 2 of the accompanying drawings.
After the above-described Carlson method, some types of electrophotography have been proposed which use a photosensitive medium having an insulating layer on a photoconductive layer surface and in which electrostatic image is formed on the photosensitive medium. In one of these types of method, there can be formed electrostatic images which are stable even if exposed to light at a high contrast on the photosensitive medium. Such method is disclosed in detail in U.S. Pat. No. 3,666,363 or U.S. Pat. No. 4,071,361.
Such electrophotography is illustrated in FIG. 3 of the accompanying drawings wherein column I schematically shows the variation in the charge of the photosensitive medium, column II shows the variation in surface potential of the photosensitive medium, and column III shows the variation in the surface charge density of the photosensitive medium.
The photosensitive medium 1 basically comprises a conductive layer 11, a photoconductive layer 12 and an insulating layer 13 (transparent). In the process I, primary charging is effected to uniformly charge the surface of the photosensitive medium by a primary charger C1, to the positive polarity if the photoconductive layer is N-type, and to the negative polarity if the photoconductive layer is P-type. Next, in the process II, the photosensitive medium is exposed to original image whose density is linearly varied and simultaneously therewith, corona charge opposite in polarity to the primary charge or AC corona discharge is imparted by a corona discharger C2 to reduce the surface potential while causing a charge density variation corresponding to the original image. In the process III, the whole surface of the photosensitive medium is uniformly exposed to light to release the charge in the vicinity of the interface between the photoconductive layer and the insulating layer corresponding to the dark region of the original image to thereby effect whole surface exposure representing the surface potential of high contrast. The electrostatic latent image thus formed is subjected to the development, image transfer and fixation as in the above-described method, thereby providing a copy.
The copy formed by the above-described electrophotography has been sufficiently satisfactory for the office purposes. However, the method cannot faithfully reproduce originals containing grey or colors in a wide range for precision printing or artistic printing and so, it has encountered difficulties in obtaining copy images of sufficiently good quality for the precision printing or artistic printing which requires faithful reproduction of the original.
In the conventional electrophotography, light grey on an original image has been reproduced lighter on a copy while dark grey on an original image has been reproduced darker on a copy. Also, in color copying, if it is attempted to reproduce delicate color variations, the resultant copy has become cloudy in color and might have suffered from great color change with respect to the original.
Although these have been improved by improvement of toner characteristic, faithful reproduction of originals are still difficult.
The present invention has been made in view of the above-described points.